This invention relates to analyzing electrical circuit boards, e.g., to identify an integrated circuit which has failed.
In testing circuit boards it is desirable to be able to identify individual elements which have failed without having to disconnect the elements from the circuit. Circuit faults can be detected by voltage and waveform measurements, but when several elements are connected to a point it is difficult to identify, e.g., which element has short circuited to ground. Current tracing and measurement methods may locate the failed element but generally require successive measurements to be made at various points between elements, which can be difficult when conductive paths between the elements are short.
In a U.S. patent application entitled "Analyzing Electrical Circuit Boards" (Ser. No. 879,881, filed Feb. 21, 1978) recently filed by Richard P. Davis (hereby incorporated by reference), there is disclosed Davis invention of providing a probe having at least two contact tips close enough to each other to simultaneously contact a lead of a mounted integrated circuit, but spaced apart from each other sufficiently to permit measurement of electrical activity in the lead segment between the tips; and measuring the voltage drop between the two tips resulting from the flow of test current through the resistance of the lead segment, which current flow is indicative of the condition of the element. In an embodiment conceived by Davis a third tip was provided to inject a test current directly into the lead, and the voltage drop across the lead segment was made as the current flowed in one direction through the segment. In another embodiment Davis had three tips spaced so that injection could be through either outside tip, and the voltage drop could be measured between the other tips; in that embodiment Davis' tips spanned a total distance of 1/10", and were thus not close enough to reliably fit on the straight portion of a mounted IC lead. But Davis used his probe only to draw qualitative, order of magnitude conclusions about current flows, as any quantitative conclusions would depend on the resistance of the lead segments, which can vary widely from lead to lead.
In another U.S. patent application entitled "Analyzing Electrical Circuit Boards" (Ser. No. 880,056, filed Feb. 22, 1978), recently filed by Alexander Pitegoff (hereby incorporated by reference), there is disclosed Pitegoff's invention of providing a probe with at least two tips spaced generally according to what Davis taught, injecting a test signal directly into the lead via the probe, measuring the voltage drop across a lead segment produced by test current flowing into the IC, also measuring the voltage drop across a lead segment produced by test current flowing away from the IC, and determining the ratio between the internal resistance R.sub.1 of the IC on the lead and the parallel resistance R.sub.2 of the remaining IC's on the node, which ratio is independent of the resistance of the lead segment.